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Title: Visualization of classical and quantum turbulence in cryogenic fluids
Other Titles: Visualizzazione di turbolenza classica e quantistica in fluidi criogenici
Authors: Fonda, Enrico
Keywords: turbulencequantum turbulencefluid dynamicsvisualizationcryogenic fluids
Issue Date: 20-Apr-2012
Publisher: Università degli studi di Trieste
Abstract: Two different projects on the characterization of classical and quantum turbulent flows using frozen particles and cryogenic fluids are reported. The first project presents the benefits and drawbacks of using liquid nitrogen for fluid dynamics experiments compared to other test fluids. This study is focused on the visualization of high-Reynolds number flow, and presents a new technique that utilizes frozen particles as tracers. The technique would provide a cheap and easy way to produce tracers for visualization in liquid nitrogen, using common hydrocarbons or even atmospheric air. The technique has been proved viable experimentally, producing micron-sized particles when atmospheric air is injected, and particles as small as 500 nm in diameter with a mixture of nitrogen gas and methane as the seeding gas. These particles have been successfully used as tracers for both particle image velocimetry and particle tracking velocimetry. The size of the particles has been estimated using a Mie scattering model that has been verified with polystyrene latex particles of know size and index of refraction. A study of the selection of the best seeding gases, which involved estimates of the density and index of refraction of several hydrocarbons, showed that propylene and propadiene would produce the brightest faithful tracers. An analysis of the parameters attainable using liquid nitrogen as a test fluid for different experiments has been conducted. The experiments considered include grid turbulence, pipe flow, Taylor Couette flow, von Karman flow and Rayleigh-Benard convection. A discussion of the feasibility and a comparison with different test fluids state-of-the-art existing experiments is presented. The second project involves the study of superfluid turbulence in HeII. Quantized vortices can be visualized using micron-sized solid hydrogen particles as tracers. Because of their size, Stokes drag does not allow them to stay trapped on quantized vortices close to the lambda-transition, where the trapping potential is weaker. A new technique has been discovered to create and visualize sub-micron particles. Several size estimates of these nanoparticles have been made based on both optical and fluid dynamical properties. Being smaller, but not small enough to be influenced by thermal motions, the particles are more passive and are less affected by Stokes drag. Thus they stay trapped closer to transition and on faster moving vortices. The ability to create particles directly into the $HeII$ allows the visualization of the vortex dynamics at temperatures lower than ever before. Particles of different size have been used to study a thermal counterflow. For low heat fluxes, these particles can either trace the motion of the normal component or track quantized vortices when they get trapped on their cores. For high heat fluxes, the increased number of particle-vortex interactions and scattering events result in a different state in which the particles track neither the vortices nor the normal component. These observations confirm the hypothesis by Sergeev and Barenghi that the discrepancy in the experiments by Paoletti et al. and by Zhang and Van Sciver are due to different regimes of particle-vortex interactions. Analyzing the trajectories of tracers of different size for a wide range of heat fluxes, the particle-vortex interaction mechanism is investigated with a new toy model
Description: 2010/2011
NBN: urn:nbn:it:units-9393
Appears in Collections:Scienze matematiche e informatiche

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